The Hall-Heroult process was first used commercially around 1900. In this process, aluminum is extracted by electrolyzing aluminum oxide (also known as "alumina") dissolved in a molten salt bath based on cryolite, Na.sub.3 AlF.sub.6. The molten cryolite is operated at a temperature generally with the range of 950.degree.-1000.degree. C. In the electrolytic cell, a carbon lining within a crucible typically serves as the cathode, and the anodes, typically carbon, are immersed in the molten salt. The molten cryolite-aluminum oxide serves as the electrolyte solution. Heat produced, for example, by a large electric current in the cell, melts the cryolite which dissolves the aluminum oxide and maintains the aluminum being electrolyzed in the molten state in which it collects in the bottom of the cell.
The Hall process, although commercial today, has certain limitations, such as the requirement that the process operate at relatively high temperatures, typically around 970.degree. C.
The high cell temperatures are necessary to achieve a high alumina solubility. At these temperatures, the electrolyte and molten aluminum progressively react with most carbon or ceramic materials, creating problems of metal and electrolyte containment and cell design.
The high temperature salt baths of the prior art are typically enveloped in a frozen sidewall and/or frozen ledge of salt bath, which helps reduce the corrosive effects of the electrolyte and metal on the containment vessel. Maintaining a frozen sidewall or ledge, however, requires a significant heat loss from the system, and any attempt to insulate the system to significantly conserve heat loss results in the melting of the frozen ledge or sidewall.
In general, the carbon anodes are consumed in the Hall process with the evolution of carbon oxide. Practically speaking, the consumption of carbon anodes requires adjustment of the anode-cathode distance to maintain it within certain critical limits. Although 0.33 kg of carbon is theoretically required for each kilogram of aluminum produced, nearly 0.5 kilograms of carbon per kilogram of aluminum can actually be consumed, for instance by losses due to air burning and back reaction between aluminum and CO.sub.2. Purity requirements for the aluminum produced necessitate the use of high quality coke for the anodes. In the United States alone, carbon consumption for the production of aluminum is nearly 2-5 million tons per year. If an inert anode could be found to replace the carbon anodes the energy content of the coke could be saved, and O.sub.2, rather than carbon oxide would be produced at the anode. In addition, emissions of fluorocarbons and sulfur would be eliminated.
Other disadvantages of the Hall cell include sodium intercalation and formation of sodium aluminum oxide which causes heaving and cracking of the cell lining, with resulting interference in operating characteristics of the cell and shortened cell life, requiring periodic cell relining.
Numerous methods have been attempted to overcome some or all of the above shortcomings of the Hall process. While many of these methods have met with some success, none has replaced the conventional Hall process in commercial applications. One attempt has been to utilize so-called "low temperature" salt baths which allow reduced energy consumption at the expense of lower alumina solubility. For example, U.S. Pat. No. 3,951,763 discloses a low temperature salt bath and uses a carbon anode, which is consumed in the process. U.S. Pat. No. 3,996,117 adds 5% to 10% by weight LiF to the bath.
One of the drawbacks of the low temperature salt bath technology has been the realization that reduction of salt bath temperature likewise leads to reduction of alumina solubility. Attempts to overcome this problem include those disclosed in U.S. Pat. No. 3,852,173 wherein the alumina is provided with a sufficient water content to prevent anode dusting, which water content also assists in dispersing the alumina into the low temperature salt bath solution of NaF/AlF.sub.3. However, providing the water-containing alumina is an added requirement of the process and naturally incurs added expense.
Attempts at operating the salt bath at lower temperatures by using progressively lower bath weight ratios than the 1.1:1 NaF to AlF.sub.3 bath ratios typically used have been frustrated by the formation of a crust of frozen electrolyte over the molten aluminum as electrolysis proceeds. This crust drastically increases resistance at the cathode, reduces metal coalescence and causes deposition of sodium, which in turn, hampers current efficiency. Under these conditions, the cell can no longer be operated efficiently.
Various attempts have been made to utilize so-called "inert" anodes in order to improve the Hall process. See, e.g., U.S. Pat. Nos. 3,718,550; 3,960,678; 4,098,669; 4,233,148; 4,454,015; 4,478,693; 4,620,905, 4,620,915 and 4,500,406. Attempts have also been made to use inert anodes with low temperature salt baths. U.S. Pat. No. 4,455,211 discloses a low temperature salt bath of NaF/AlF.sub.3 which teaches the addition of 1% to 15% LiF and an inert anode made of an interwoven matrix. PCT Application No. WO 89/06289 discloses the use of an inert anode in connection with a metal chloride and/or metal fluoride salt bath using additives for low temperature aluminum electrolysis. However, this reference teaches the need to increase the actual anode surface area by 2 to 15 times the superficial or projected anode surface area. Such increased surface area anodes are typically fabricated, for example, by drilling numerous holes deep into the anode or using an array of plates or rods for anodes. Such anodes typically have an active surface area several times the cathode active surface area.
U.S. Pat. No. 4,681,671 discloses a low temperature salt bath which is used in conjunction with an anode having a relatively large surface area (actual or active area at least 1.5 times larger than the projected surface area) and low current density. Indeed, this reference teaches the necessity of utilizing a low current density and increased anode surface area in conjunction with low temperature salt baths and inert anodes.